Field of the Invention
The present invention relates to a laminated carbon brush for a fuel pump motor.
Description of the Related Art
Carbon brushes for fuel pump motors are used in liquid fuels and slide on commutators. The liquid fuel exists between the brush and the commutator, and electrical contacts between them are unstable. Therefore, spark discharges occur often between the brush and the commutator, particularly at the sliding end where the brush and one segment of the commutator become separated. The spark discharges make the respective sliding ends of the brush and the commutator damaged and dropped off, and therefore, the sliding surfaces of them become uneven. Then, the contacts between them become further unstable, and the larger spark discharges occur. Thus, the brush and the commutator rapidly and abnormally wear, namely, resulting in the so-called “abnormal wear.” The service life of the brush and the commutator is one of the most important functions of fuel pumps. However, because a wide variety of species of fuels have been used recently, and because the fuel pump motors have been miniaturized in size and requested high outputs, the fuel pump motors have often been used under high-load. Therefore, it is necessary to prevent abnormal wears due to the spark discharges.
The fundamental measure to the prevent abnormal wears is to suppress the occurrence of spark discharges. In order to prevent abnormal wears, various additives into the brushes or the commutators have been studied. However, the improvement in the wear resistance and the suppression of the spark discharges are not the same.
The related prior art will be described. Patent Literature 1 (JP 2012-50276A) proposes a two layer resin bonded laminated carbon brush comprising the lower resistivity layer and the higher resistivity layer. Here, “resin bonded” means that an uncarbonized synthetic resin acts as the binder. Carbon brushes have anisotropy in the resistivity determined by the press direction during molding; generally, the resistivity along the press direction is high, and the resistivity in a plane perpendicular to the press direction is low. The carbon brush according to Patent Literature 1 slides on a cylindrical commutator; its resistivity along the axial direction of the commutator is made high, and the resistivity along a direction perpendicular to the axial direction is made low. The resistivity of the higher resistivity layer is 71,000 μΩ·cm along the circular direction of the commutator, for example, and is 760,000 μΩ·cm along the axial direction, for example. Note that the carbon brush according to Patent Literature 1 is not for fuel pump motors, and the spark discharges are not taken into consideration.